Study investigates reducing radiation dose in prostate stereotactic body radiotherapy

Additional radiation dose optimization to the rectal wall (R_wall) past that usually used in urethra-sparing prostate stereotactic body radiotherapy (SBRT) does not compromise target dose homogeneity in patients with and without endorectal balloon (ERB), according to a new study in Radiation Oncology.

“Both conventionally fractionated radiotherapy and SBRT studies have shown that higher dose to the rectum correlates with increased rectal toxicity,” wrote researchers, led by Angèle Dubouloz, DSc, Geneva University Hospital, Geneva. “Minimizing the dose to the rectum, over the whole range from low to high doses, could impact the quality of life.”

SBRT is an emerging, safe treatment for prostate cancer. Particular clinical interest is paid to extreme hypofractionated treatments due to low α/β ratio values for prostate tumors compared with those of the nearby organs at risk (OAR), such as the rectum, urethra, and bladder.

Currently, OAR dose-constraint guidelines for SBRT have yet to be fully elucidated. In this study, researchers carried out a dosimetric comparison in the context of a randomized phase 2 trial examining urethra-sparing SBRT for prostate cancer,

“The principal goal of the study was to determine the optimal strategy for minimizing the low-to-intermediate-dose regions of the R_wall,” the authors wrote, “with the secondary goal of assessing the potential dosimetric benefit for the R_wall of using an ERB compared to no-ERB.”

In total, 10 prostate cancer patients, simulated both with and without ERB, were to be administered 36.25 Gy to the planning treatment volume (PTV) and 32.5 Gy to the urethral planning risk volume (uPRV).

The team optimized reference plans both with and without ERB, taking into consideration PTV and uPRV objectives and OAR dose constraints. They then further optimized these plans via a standardized stepwise approach to further decrease dose constraints to the R_wall in the low to intermediate range in five steps to yield paired plans with and without ERB (Vm₁-Vm₅). The researchers analyzed homogeneity indices for the PTV and the uPRV, as well as the Dice similarity coefficient (DSC) for the PTV.

The authors found that beginning with personally optimized plans, R_wall sparing can be ameliorated in four optimization steps (Vm₄) without compromising PTV or uPRV coverage. This approach, however, did result in more monitor units and slightly increased dose to the femoral heads.

The investigators suggested that the administration of additional MU and its effect on beam-on time could be resolved with flattening filter free beams to decrease treatment delivery time.

“By pushing down the R_wall DVO constraints to the limit (ie, until the TPS [treatment planning system] is unable to optimize without impairing the PTV coverage), we observed that treatment plans with and without ERB gave similar results in the low-to-intermediate dose range,” the authors wrote.

This finding is also supported by the work of previous researchers.

“The main outcome of this work was to establish that one optimal technique for reducing the low- to- intermediate dose to the R_wall was a step-wise optimization approach,” the authors concluded. “Despite the inherent limitations of our study, we were unable to demonstrate that the use of an ERB allows additional low-to-intermediate dose reduction to R_wall.”